Quantum key distribution (QKD) enables two parties to establish a secure key at a distance, even in the presence of one or multiple eavesdroppers. The key can then be used to secretly transmit information using unconditionally secure one-time pad encryption. Unconditional security is achieved by the laws of physics rather than by assumptions about the computational abilities of the eavesdropper. Various QKD protocols have been proposed and implemented. A first protocol due employed polarization states of photons passed between the two parties. There has been growing interest in schemes employing photons in Hilbert spaces of high dimension, resulting in a potentially very large alphabet size. Different degrees of freedom have been considered, including polarization, time, and spatial modes.
What is needed is a protocol that allows two parties to generate their secure key at the maximum rate allowed using the time-energy basis. Moreover, a protocol is needed that is compatible with fiber-based dense WDM (DWDM) systems commonly used in classical fiber communications. It is also desirable to have an extremely compact, stable, and scalable platform for the protocol's implementation.